Instruments that are used for various diagnostic purposes such as the examination of eyes, ears, noses, and throats and other tissue structures often have a light source contained within the instrument enabling it to emit a light beam so as to illuminate the structure under observation. This is both convenient and practical. An image detection means (whether the human eye or otherwise) is used to register and interpret any resultant returning light.
Such an instrument may be functionally broken down into a number of smaller modules which each perform a discrete technical operation. Most instruments contain at least four modules in their construction, however extra modules may be added to enable additional functions to supplement those of the core modules.
The first module is an electrical power supply, which may be powered entirely by the mains with or without a transformer low voltage conversion, or a battery (either standard cells, rechargable cells or a combination of both). The latter battery type is preferred, especially the rechargable type especially when trickle charged because it enables the instrument to be portable and ready for use.
The second module is a means of controlling the electrical power which should be capable of handling the often high currents generated by the power supply whilst also being capable of varying the electrical power according to requirements. Conventional instruments utilise a heavy-duty ganged rheostat, which acts as an on/off switch and a current limiter thereby varying the electrical power which flows through the light source. The rheostat is typically of a low value (e.g. max resistance of ˜8 ohm when first on, down to ˜0 ohms in an approximately linear manner).
The third module is a means for generating light which typically is a light source element or elements. Conventional instruments typically use incandescent filament based bulbs that draw a large current (e.g. ˜0.2 A to ˜0.8 A) to enable an acceptable amount of visible light to be produced. Such bulbs are often of the halogen gas filled type which are usually of small size and are of a specialised nature.
The fourth module is a means for transforming the light, which may comprise lenses, filters, collimating means and other means to transform the light. This module is usually specialized for examining the structure under observation (e.g. eyes, ears etc).
An extra module that is commonly employed is a means of transforming any returning light from structure under scrutiny. This is often a rack of interchangeable lenses enabling fine focus of the light onto the image detection means.
The modules are typically combined together so as to produce a convenient, often handheld, combined instrument that is flexible and easy to use, whilst also enabling the instrument to be disassembled to enable replacement of modules (for example replacing expired bulbs). Although it is possible to change the fourth module to form a different instrument, in practice instruments are not usually used in this way and dedicated instruments are used for specific tasks.
To illustrate this, two widely used diagnostic instruments, namely an opthalmoscope (OS) and retinoscope (RS), will now be described. Both the OS and RS project a beam of light, which is used for examining the eye. Currently available handheld OS and RS have a dial (which is part of the rheostat control module) on the handle of the instrument, which enables light output to be smoothly and precisely varied. Such variability of the light beam is preferable for diagnostic purposes. An OS images a bright, small filament via lenses, mirrors and filters to produce a beam of light which is suitable for illumination of the eye. An RS images a bright, small, thin elongated filament in a similar way. The image detection means (usually the human eye) then receives any returned light, possibly via any fine focus lenses. In a typical clinical environment such an instrument may be in intermittent but repetitive use all day.
Although diagnostic instruments as described above based on filament bulb technology are widely used they suffer from numerous problems:
Filament bulbs have a limited life and eventually fail. In a typical clinical environment, depending on usage, this may occur as often as every six months. As a consequence such instruments are typically designed to allow the insertion of replaceable bulbs at periodic intervals by allowing separation of the instrument followed by reassembly.
Filament bulbs get hot during use due to dissipation of large amounts of infrared radiation which in turn is due to their inherent luminous inefficiency. Therefore such instruments are typically designed with a metal, or other heat conducting material, casing to act as a heat sink. A filament bulb will also usually have a container of similar construction, thus also acting as a heat sink.
Large amounts of infrared radiation is emitted in addition to useful visible light. As examination can often take a prolonged period of time this can often prove uncomfortable to the subject and/or have a possible deteriorating effect on the tissues under scrutiny. Thus, an infra red safety filter is typically incorporated within the fourth module to provide a ‘cool’ beam.
Due to the low luminous efficiency of filament bulbs they consume a high quantity of electrical power. Thus, for the instrument to be useful in a portable mode of operation, large batteries are needed. Additionally, a control apparatus capable of handling high power (e.g. a heavy rheostat) is needed.
The spectral light distribution of a filament bulb has a low colour-temperature (i.e. yellowy light). Some instruments incorporate a colour-correction filter to alter the light distribution. This is required to provide an improved colour-rendering-index factor, which is beneficial for analysis of biological tissues or other detailed tasks.
An incandescent filament emits its light flux effectively in all directions. This light usually has a high degree of coherence due to the small filament size. An optical condenser lens system is typically used to image this light source. To be efficient the condenser lens system should be close to the light source (to reduce light wastage) which necessitates it to be powerful in order to direct a portion of the light ‘forwards’. This ‘forwards’ light is then usually projected by a field lens to a semi-silvered or sight-hole based, or other mirror. Much of the emitted light flux is thus not utilized and is absorbed within the instrument. There is usually no space in such instruments to have a reflector arrangement as an alternative to this.
Additionally, as bulbs for use in such instruments are required to have a small filament size (for efficient optical imaging purposes), they must be very accurately centered due to the requirements of the complex lens system.
The filament is surrounded by a fragile glass envelope. The operator must be very careful to avoid touching the glass, as grease from the fingers can cause cracks to develop on the glass envelope contributing to degradation and ultimately reducing the life of the bulb.
It is the aim of the present invention to overcome at least some of the above problems.